The overal aim is to elucidate the mechanisms of drug delivery and polymer degradation from biodegradable polymers, to compare in vivo/in vitro situations and to find out the cause for differences, to characterize the inner environment of the delivery system to which drugs are exposed prior to their release and how this environment does change with time when the delivery system is exposed to a buffer solution or implanted in animals, to investigate the influence of incorporated drugs on the inner environment of the polymer, to obtain spatial information on the processes in the polymer by means of sandwhich-type tablets containing different isotopes of the nitroxide, and to investigate the influence of the pharmaceutical formulation and the route of administration on the in vivo fate of the drug and the delivery system.